Ultrasonic imaging is commonly performed to provide an image of structure internal to an object. For example, it is common to perform ultrasonic examination, e.g., of pregnant women by disposing a multiple-element ultrasonic transducer against the abdomen and sequentially exciting the elements of the transducer. Pulses of ultrasonic energy are then transmitted inwardly, and are reflected at interfaces between structures of differing acoustic impedance. The reflected energy is detected by the transducer elements, which then provide electrical signals responsive to the detected energy. These signals can be processed to yield a visible image of the interfaces, that is, of the outlines of various structures within the abdomen.
Ultrasonic examination is also commonly carried out endoscopically, that is, using transducers mounted on probes inserted into the patient's body through a naturally occurring or surgically formed portal so as to be juxtaposed to an organ or other structure of interest. Ultrasonic examination is also performed in industrial applications, e.g., for non-destructive examination of manufactured products and the like.
A number of scanning and image display techniques have become generally standardized. These include the "A-scan", wherein a focused or "pencil" beam is directed into the object of interest. The reflected energy, that is, "echoes" are detected and displayed, by a "trace" plotting the amplitude of the received energy signal on the vertical axis and time after transmission on the horizontal axis, such that the distance of features in the trace from one edge of the display indicates the relative depth of the reflecting interfaces. To provide three-dimensional information describing internal structure using the A-scan technique requires performing such examinations at spaced locations along a number of generally parallel lines, and mentally interpolating the results; this process is too difficult for most practitioners.
The A-scan process has been largely supplanted by the "B-scan" technique, wherein a two-dimensional image, i.e., an image of a cross-sectional "slice" is formed An elongated multiple-element transducer is commonly used to provide a series of parallel "pencil" beams focused by sequential addressing or phase-steering techniques. A focused beam may also be physically steered. "Lines" corresponding to energy reflected from structure along the path of successive pencil beams extending into a particular portion of the object are displayed side-by-side, while the brightness of the display is varied responsive to the strength of the reflected signal. The images formed each correspond to a cross-sectional view or two-dimensional "slice" extending into the object to be imaged. If the transducer is moved slowly along a line perpendicular to the image plane while a sequence of such images is formed, one can mentally interpolate changes in the shapes of the interfaces, that is, in order to obtain a three-dimensional mental image. Although this technique is workable for highly-skilled practitioners, clearly it would be desirable to provide a true three-dimensional image, capable of being recorded or displayed to others. Moreover, a three-dimensional image would be highly useful in determining the position of an implement, e.g., monitoring the motion of a biopsy needle approaching a tumor.
The prior art suggests several techniques for providing a three-dimensional image employing ultrasonic energy. For example, see U.S. Pat. Nos. 4,747,411 and 4,821,728 to Ledley, disclosing the "reconstruction" of three-dimensional information by automated comparison of two-dimensional "slices" extending into the object. The reconstructed information is used to generate two images exhibiting relative parallax corresponding to the two images formed by one's eyes, that is, with foreground objects being displaced from one another in the two images by a greater amount than background objects. The two images may be displayed side-by-side for simultaneous stereoscopic viewing, for example, on adjoining video screens, as disclosed by Ledley, or may be displayed alternatingly on a single screen, together with means for ensuring that each of the viewer's eyes "sees" only the corresponding image. For example, the image intended for one eye may be polarized in a first sense, and that intended for the other eye polarized in the opposite sense, and the viewer provided with "eyeglasses" fitted with corresponding polarizing filters. A substantially similar effect can be achieved using differential coloring of the two images and colored filters.
The prior art has typically generated such pairs of images for three-dimensional display by forming two images using ultrasonic sources and detectors at spaced viewpoints, that is, corresponding to the viewer's eyepoints. See the Ledley '411 patent at col. 5, lines 51-55. This is, of course, the intuitive first approach to the problem of providing three-dimensional images. However, there are numerous deficiencies to such an approach, one being that the image displayed is constrained to represent objects being imaged as if seen from a viewing location corresponding to the actual location of the sources and detectors of the ultrasonic energy. In many circumstances the transducer is constrained by anatomy from being placed at a distance from the structure to be imaged suitable for normal viewing. Furthermore, as acknowledged by Ledley, the number of sources and detectors that can be employed in a given scanning plane is limited, reducing the effective signal-to-noise ratio.
Ledley suggests (col. 6, lines 16-42) that the same result can be obtained using a single source/detector pair swept physically through a plurality of angles and processing the returned signals to "develop data representing forward projections of the object . . . onto the! image plane . . . ". However, this arrangement would also seem to suffer the disadvantages mentioned above, and would further require a mechanical device to provide the scanning motion. The latter not only involves additional complexity and expense, but also requires that a number of images be formed over a period of time; this imposes the requirement that the object be stationary, precluding imaging of a living heart, for example.
Other prior art patents generally relevant to the subject matter of this invention include U.S. Pat. No. 4,028,934 to Sollish, showing two mechanically scanned cylindrically focused elongated transducers used to form images with respect to image planes forming an angle to one another. See FIGS. 6 and 7. The two images are used to provide a perspective view rather than cross-sectional tomograms. See FIG. 3. The images are displayed on two screens or slides and viewed stereoscopically. Sollish also suggests that a number of B-scan images can be processed to provide a holograph displaying three-dimensional information, although this process would be time-consuming and impractical.
A number of patents, including U.S. Pat. No. 4,097,835 to Green, U.S. Pat. No. 4,924,869 to Takeuchi et al, U.S. Pat. No. 5,090,411 to Higuchi, and U.S. Pat. No. 5,050,611 to Takamizawa et al, show simultaneously generating Doppler blood flow data together with B-scan tomographic information. The Green patent shows semi-circular piezoelectric transducers with focusing elements moved in opposite directions along parallel linear paths.
U.S. Pat. No. 5,186,175 to Hirama et al. shows a system wherein a large number of transducers are arranged in lateral and azimuth directions to form a two-dimensional array.
U.S. Pat. No. 4,100,916 to King, which was reissued as Re. 30,397, shows an apparatus for three-dimensional ultrasonic imaging in which electrical sparks are used to provide the ultrasonic energy. The system is used only external to a body structure to be imaged.
U.S. Pat. No. 4,846,188 to Yoshioka shows a system wherein images synchronized to a patient's heart beat are sequentially collected, evidently to provide a "movie" of images.
U.S. Pat. No. 4,787,394 to Ogura shows a system wherein two ultrasonic transducers are used to form two tomograms to monitor the destruction of a calculus such as a kidney stone, using a third beam emitter as the source of destructive energy.
U.S. Pat. No. 4,733,562 to Saugeon shows an ultrasonic array divided into subarrays for providing a steered ultrasonic beam.
Finally, U.S. Pat. No. 3,156,110 to Clynes shows generation of energy at three different ultrasonic frequencies for reflection from different sorts of structures within the body.
Most prior art directed to ultrasonic imaging has attempted to image as thin as possible a cross-sectional "slice" through the structure of interest, as the thickness of the slice limits the resolution of the image. The usual "B-scan" ultrasonic imaging techniques employed require many such images to be formed and mentally interpolated in order to convey three-dimensional information. The Ledley patents attempt to eliminate the necessity of mentally interpolating a series of "slices" by reconstructing the slices to generate images exhibiting parallax for stereoscopic display. However, the Ledley approach introduces further difficulties, as noted above. Furthermore, if a particular feature of the structure does not happen to coincide with any of the slices, that feature will be omitted entirely from the image. It would be highly preferable to provide a three-dimensional image of an entire volume of an object of interest.
Howry et al, "Three-Dimensional and Stereoscopic Observation of Body Structures by Ultrasound" J. Appl. Physiology 9, 304-6, (1956), proposes stereoscopic viewing of two `pictures`, each comprising a sum of a number of cross-sectional views of an object formed using ultrasonic techniques. The cross-sectional views making up each picture are formed sequentially, by moving a transducer along a vertical line perpendicular to a line connecting the transducer and the object. The cross-sectional views thus formed at each position of the transducer along a first line are simultaneously displayed, and are effectively summed to form the first of the pictures. Each of the views being summed is displayed with gradually varying vertical scale, such that the summed picture effectively corresponds to a perspective view. A second set of similarly formed views from transducer locations extending along a second vertical line spaced from the first is then formed and summed similarly. The two pictures are then displayed side-by-side for stereoscopic viewing, such that parallax in the two pictures yields a three-dimensional image.
Henderson, "A Stereo Sonar for Object Examination" 1992 IEEE , Ultrasonics Symposium, 1151-54, discusses a method of displaying two sonar images for exhibiting three-dimensional information. An elongated multiple-element transducer arranged transverse to a line connecting the transducer and the object is employed to form first and second sonar images at a single location; the transducer is tilted typically through 10.degree., after formation of the first image Both images include energy reflected from reflecting surfaces on the half of the object toward the transducer as if seen from above, such that reflecting surfaces on the upper and lower portions of the object overlap in each image. The images are displayed for stereoscopic viewing, that is, each image is "seen" by one of the viewer's eyes. Parallax in the two images allows the viewer to synthesize the depth information and separate reflectors on the upper and lower surfaces of the object.
Smith et al, "High-Speed Ultrasound Volumetric Imaging System--Part I: Transducer Design and Beam Steering" IEEE Trans. on Ultrasonics, Ferroelectrics and Frequency Control, 38, No. 2, 100-108 (1991) and von Ramm et al, "High-Speed Ultrasound Volumetric Imaging System--Part II: Parallel Processing and Image Display", id. at 109-115, disclose a system for three--dimensional ultrasonic imaging wherein a two-dimensional, planar array of transducer elements is used to transmit ultrasonic energy into a generally pyramidal volume of an object to be imaged. Energy reflected from structure at given depths from the transducer array in the volume is imaged in a similar pattern, such that the display is of a cross-section parallel to the surface of the object. If two images taken at relatively spaced or angled viewpoints are displayed for stereoscopic viewing, 3-D relationships may be perceived. A number of differing methods of conveying 3-D information to the viewer are disclosed at 113-14, including alternating display of stereoscopic images on a single display, using polarization techniques to ensure that each of the viewer's eyes sees the only the appropriate image, or by color-coding energy reflected from different depths in the volume.
Entrekin et al, "Real-time 3-D ultrasound imaging with a 1-D `fan beam` transducer array", SPIE V. 1733, 264-272 (1992), and Entrekin et al. U.S. Pat. No. 5,305,756 refer to the results of Smith et al. and von Ramm et al. discussed above, but disclose modification of a conventional elongated, multiple-element (i.e. "1-D") transducer array as used for linear or phased-array B-scan ultrasonic examination. Conventionally, in order to provide a narrow beam and improve the resolution of the image, a transducer array is employed with a convex RTV silicone rubber lens over the active surface of the transducer array; as the speed of sound in RTV rubber is slower than in tissue, this array/lens combination provides a collimated beam in elevation, i.e., one that is narrow in the plane including the axis of elongation of the array. Such a transducer may be rotated about its axis through an angle of typically 60.degree. to form a conventional 2-D image; that is, the reflected energy detected at each of a plurality of angularly-spaced positions of the transducer is displayed line-by-line to form the B-scan image.
According to the modification disclosed by Entrekin et al, the narrow beam conventionally provided is replaced with a beam diverging in elevation, so that a wedge-shaped volume of the object, with the apex of the wedge extending along the surface of the transducer array, is illuminated with ultrasonic energy. An image is then formed as in the conventional B-scan technique, except that each line of the image includes energy reflected from structure within the corresponding wedge-shaped volume. The wedge-shaped beam may be provided by replacing the conventional convex RTV rubber lens by a concave lens of the same material, so that the beam diverges in elevation, by forming the transducer with a convex outer surface, or by electronically forming a diverging beam using a two-dimensional transducer array. See col. 5, line 54 through col. 6, line 29 of the Entrekin et al. patent.
Images included in the Entrekin et al. paper illustrate that a quasi-3-D effect is provided. However, as acknowledged by Entrekin et al. (see "5. Discussion"), their method has several significant limitations. Chief among these is the fact that all reflectors at the same range in elevation are superposed in the display, rendering the actual location of the reflectors ambiguous. Where the object is in motion, this ambiguity can be resolved by the eye of the viewer. However, in many circumstances, this fact limits the utility of the Entrekin et al. system to imaging the surfaces of objects surrounded by fluid.